Method And Apparatus For Spacecraft Gyroscope Scale Factor Calibration

A method for calibrating a scale factor of a spacecraft gyroscopic angular rate sensor, comprising the steps of:

• a) predetermining a frequency and amplitude of sinusoidal spacecraft attitude motion that is within required attitude accuracy, attitude stability and attitude control system actuator control authority margins such that a period of the sinusoid is equal to a data sample period of the spacecraft gyroscopic sensor times two raised to an integer power;
• b) imparting sinusoidal motions of the predetermined amplitude and frequency to the attitude of the spacecraft, one axis at a time;
• c) computing an angular rate that is compensated for bias drift and misalignment using data from a gyroscopic angular rate sensor output;
• d) integrating said compensated angular rate to obtain a time profile of relative gyroscopic sensor attitude motion about the axis of imparted motion;
• e) processing said time profile of relative gyroscopic sensor attitude motion about the axis of imparted motion using a Fast Fourier Transform to obtain a gyroscopic sensor Fourier coefficient corresponding to the sinusoid frequency;
• f) computing a time profile of relative star tracker attitude motion about the axis of imparted motion over the same time span as the integrated compensated angular rate using data from a star tracker of the spacecraft;
• g) processing said time profile of relative star tracker attitude motion about the axis of imparted dither motion using a Fast Fourier Transform to obtain a star tracker Fourier coefficient corresponding to the sinusoid frequency;
• h) calculating a single-point estimate of a gyroscopic sensor scale factor by calculating a ratio of the gyroscopic sensor Fourier coefficient computed in step e) to the star tracker Fourier coefficient computed in step g);
• i) repeating steps a) through h) for the selected axis and calculating an arithmetic mean of the single-point estimates of the gyroscopic sensor scale factor;
• and
• j) repeating the steps a) through i) for a second axis.

An apparatus for calibrating a scale factor, comprising:

• an angular rate sensor;
• means for imparting an excitation to the angular rate sensor, wherein said excitation comprises a periodic motion having a predetermined fundamental frequency and a predetermined amplitude;
• means for collecting angular rate measurements from the angular rate sensor during the imparted excitation;
• means for computing an amplitude of an integral of the collected angular rate measurements at the fundamental frequency of the imparted excitation;
• means for computing an expected value of the amplitude computed by the means for computing an amplitude independently of the angular rate measurements;
• and
• means for computing an angular rate sensor scale factor estimate as a ratio of the amplitude to the expected value.

An apparatus for controlling an attitude of a vehicle, comprising:

• a gyroscope;
• a star tracker;
• an attitude command generator, wherein said attitude command generator generates a multi-axis attitude command signal;
• a dither signal generator, wherein said dither signal generator generates a dither signal;
• a summer having inputs connected to said attitude command generator and said dither signal generator, wherein said summer receives said multi-axis attitude control signal and said dither signal and outputs a modified attitude command signal;
• an attitude determination and control module comprising a processor, wherein
• said attitude determination and control module receives data from said gyroscope and said star tracker and receives said modified altitude command signal from said summer,
• said processor uses said gyroscope and star tracker data to calculate an estimate of true vehicle attitude, and
• said attitude determination and control module generates torque commands to cause a true attitude of said vehicle to track a modified command attitude.